An Erythrocyte-specific Protein That Binds to the Poly(dG) Region of the Chicken Beta-globin Gene Promoter

The promoter region of the chicken adult beta-globin gene contains a sequence of 16 deoxyguanosine residues located at a nucleosome boundary in tissues where the gene is inactive. In definitive erythrocytes that express the beta-globin gene, the nucleosome is displaced, the G-string and adjacent sequences are occupied by sequence-specific DNA-binding proteins, and a nuclease hypersensitive domain is generated in this region. To gain insight into the role of the G-string in this series of events, we have examined the proteins that bind to it. Using the gel mobility shift assay and a monoclonal antibody that blocks specific binding to the G-string, we have identified a specific protein, BGP1, that is found only in chicken erythroid cells and appears at the same time, or shortly before, the changes in chromatin structure. The antibody interacts strongly with BGP1 and cross-reacts weakly with Sp1. Although both BGP1 and Sp1 require Zn2+ for their DNA-binding activity, these proteins differ in their binding-site specificities, chromatographic properties, and molecular weights. In contrast to Sp1, which is found in a wide variety of cell types, BGP1 is restricted to erythrocytes and is most abundant in definitive erythrocytes. Thus, its presence corresponds to the tissue- and stage-specific occupancy of the G-string in vivo.